In lost foam or evaporable foam casting, a pattern is produced from a polymeric foam material, such as polystyrene, and has a configuration identical to the metal article to be cast. A porous ceramic coating is applied to the outer surface of the pattern. One or more patterns may be located within an outer mold or flask, where a polymeric foam gating system connects the pattern to a sprue. The space between the pattern or and the flask is filled with a finely divided inert material, such as sand, and the material also fills the internal cavities within the pattern.
In the lost foam casting process, molten metal is fed from a pouring cup into the sprue and the heat of the molten metal decomposes the polymeric foam material of the gating system, as well as the pattern. The molten metal occupies the void created by the decomposition of the foam material and the products of the foam decomposition pass through the porous ceramic coating on the pattern and become trapped within the interstices of the sand. In practice, when casting large objects such as engine blocks, and when utilizing 5 to 50 atmospheres of pressure, it takes approximately 30–60 seconds for the molten metal to decompose and occupy the polymeric foam pattern. When the molten metal solidifies, the resulting cast article has a configuration identical to the polymeric foam pattern.
It is recognized in the art that the application of external pressure to the molten metal before solidification is completed aids in the interdendritic feeding of the molten metal during solidification and prevents both the precipitation of hydrogen porosity and the formation of microporosity in the final product. This pressurization is commonly accomplished by placing the flask and its content, including the pouring cup, into an outer pressure vessel.
However, in the current state of the art, pressurization of the flask contents is closely controlled. This is done because too rapid of a pressurization rate leads to failure of the ceramic coating. When the ceramic coating fails, metal penetration defects appear in the cast product. The failure results from the occurrence of a large  pressure gradient across the molten metal front that appears upon the sudden application of pressure. For example, when pressure is suddenly applied to the vessel, the pressure in the liquid metal rises rapidly. The velocity at which the pressure rises in the metal is at the speed of sound in liquid metal. At the same time, it is recognized that the pressure rise in the finely divided inert material is dependent upon the size and shape of the material utilized. Generally, the rise in pressure in the liquid metal is much faster than the rise in pressure in the finely divided inert material. This disparity in rate of pressure application causes a pressure gradient to appear across the molten metal front. If the pressure differential of the pressure gradient exceeds a threshold value when the molten metal front reaches the ceramic coating, then the ceramic coating will fail. When the ceramic coating fails, metal penetration of the coating results, and the casting finish takes on the surface of the sand.
Conversely, if the pressurization rate of the vessel is slow, the benefits of applying the pressure may not be realized in all portions of the casting. Thus, the casting would not see the benefits of reduction in gas porosity due to hydrogen precipitation, improved delivery during the initial stages of solidification and improved interdendritic feeding during the last stages of solidification.
Another drawback of traditional pressure application methods is that the proper application of pressure requires a very narrow processing window. This results in limitation on pressurization rates, limitations on the size and distribution of the finely divided inert material, and often results in unacceptable cast products that are scrapped. Furthermore, the narrow process window creates restrictions on manufacturing processes that create inefficiencies.